The present disclosure pertains generally to imaging systems and imaging methods, e.g., microwave and millimeter wave energy based imaging, using probes.
In the art of microwave tomography (MWT), an object of interest (OI) is illuminated by microwave energy and the scattered fields are collected outside the OI. The collected scattered fields may then be used to reconstruct qualitative, and possibly quantitative images, or interior maps, of the OI that include its location, geometry, shape, and dielectric properties. The ability to provide quantitative imaging and to utilize non-ionizing radiations associated with MWT make MWT a good candidate for use in many novel applications such as non-destructive testing in industrial applications, non-invasive imaging of biological tissues, remote sensing, geophysical survey of underground objects, and other security and military applications.
Due to the inherent non-linear and ill-posed behavior of the inverse scattering problem used in MWT, a substantial amount of electromagnetic scattering data may need to be collected in order to ensure a robust inversion and quantitatively-accurate image. The need for more data can be satisfied by several approaches such as, e.g., increasing the number of data acquisition points, using different frequencies, etc.
One advantageous approach to collect data for MWT may use co-resident antennas located at a region outside the OI, which may be referred to herein as a measurement domain. Each antenna may be successively activated as a transmitter to illuminate the OI from different angles. For each active transmitter, the electromagnetic field scattered by the OI, or the scattered field, is measured with the remaining antennas. The voltage measured by the receiving antennas may be used to infer the electromagnetic field impinging directly on each receiving antenna. This type of data collection approach may be referred to as a “direct” measurement approach.
A direct system 20 is illustrated in FIG. 15. As shown, an OI 10 is surrounded by a plurality of transmitting and receiving antennas 22. To image the OI, an antenna 22 (the leftmost antenna as shown) may deliver electromagnetic energy 12 (e.g., microwaves) having a selected polarization to the OI 10. The scattered field resulting from the electromagnetic energy impinging on the OI 10 may be collected by one or more of the antennas 22 that are not delivering electromagnetic energy to the OI 10 (e.g., all the antennas 22 except for the leftmost antenna, etc.). The signals received by the antennas 22 may be used to reconstruct an image of the OI within a pre-defined imaging domain 32.
The measurement domain 30 may not be precisely determined because all fields around the antennas 22 may contribute to the received voltage. Further, as described, all polarizations of the electromagnetic energy delivered by the transmitting antenna 22 may contribute to the voltage received by each of the receiving antennas 22 of this exemplary direct system 20.